Electric ignition systems are used for the ignition of gasoline engines. A conventional example is a current interrupting type contactless ignition device shown in FIG. 1. This comprises a trigger circuit A, a primary short-circuiting current switching controlling circuit B controlled in its operation by this trigger circuit A, an electronic protecting circuit C, an ignition coil D and a spark plug E. The trigger circuit A, switching controlling circuit B and electronic protecting circuit C are connected in parallel to lines l.sub.1 and l.sub.2 at both ends of the primary winding of the ignition coil D. The trigger circuit A, comprises a series circuit consisting of resistances 1 and 2 and a capacitor 3 and a series circuit of resistances 4 and 5 connected in parallel with each other. Between the connecting middle point of the resistances 1 and 2 and the line l.sub.2, is a series circuit consisting of a programmable unijunction transistor 6 (abbreviated as PUT hereinafter) and a resistance 7. The gate of the PUT 6 is middle point of the resistances 4 and 5.
The switching controlling circuit B, comprises a resistance 8 set in series to the collector of a switching transistor 9 the emitter of which is connected to line 12. The base of the switching transistor 9 is connected to the connecting middle point of the PUT 6 and resistance 7. A primary short-circuiting current interrupting transistor 10 is connected by its collector and emitter respectively to the lines l.sub.1 and l.sub.2 and by its base to the connecting middle point of the above mentioned resistance 8 and the collector of the switching transistor 9.
The electronic protecting circuit C consists of a diode 11 and resistance 12 connected in series between the lines l.sub.1 and l.sub.2 and has the negative current of the primary short-circuiting current bipassed so as not to be a burden on the electronic and particularly the transistor 10.
In such a current interrupting type contactless ignition device, a voltage induced in the primary winding 13a of the ignition coil D on starting the engine will be applied between the lines l.sub.1 and l.sub.2 and an induced current I.sub.o will be shunted as currents I.sub.1, I.sub.2 and I.sub.3 to the respective branch circuits. Further, the operating current of the PUT 6 will be determined by the potential at the connecting middle point of the above mentioned resistances 1 and 2 and the potential at the connecting middle point of the resistances 4 and 5. Therefore, the magnitudes of the currents I.sub.3 and I.sub.4 are so set that the PUT 6 may be ON at or a little past the peak of the primary short-circuiting current of the ignition coil D. Then, when the PUT 6 is ON, a current will flow between the base and emitter of the switching transistor 9, therefore this switching transistor 9 will be ON and the transistor 10 will be OFF. Therefore, the above mentioned primary short-circuiting current will be quickly interrupted, a maximum level high voltage will be generated in the secondary coil 13b of the ignition coil D and a spark will be generated in the spark plug E. Thus, the gaseous mixture will be burned and the engine will be driven.
However, in the contactless ignition device of such formation, when the rotation of the engine is gradually increased after the engine starts, wherein the PUT 6 had been on at or a little past the peak of the primary short-circuiting current passing through the transistor 10, the timing by which it is set ON will be gradually delayed by the resistances 1 and 2 and the charging and discharging time constant of the capacitor 3. Therefore the timing of by which the primary short-circuiting current is interrupted will be delayed and the ignition time of the engine will be delayed as shown in FIG. 2, wherein the reference symbol .theta..sub.o denotes a proper ignition time of the engine. When the resistances 1 and 2 and the charging and discharging time constant of the capacitor 3 are properly selected, the operation timing of the PUT 6 is able to be advanced. However, there have been problems that, in such case, the engine will rotate in excess and therefore the ignition coil D will seize and that, as shown in FIG. 3, the ignition time of the engine will be somewhat delayed during the period from the starting time to the normal rotation time.